Any question answered...

It just has a copper coloured dome

Police stations will happily take any rounds, firearms or other weaponry. A full-time police station mind.

That bullet is a lead core enveloped in a Full Metal Jacket hence the name of the film. Military rounds are it’s dictated by The Geneva Convention etc. if the tip is soft lead thats designed to really mess up what it hits.
A school friend was s gunsmith he had
a live German “potato masher” grenade brought in.

I would have thought you would enjoy calculating the kilometres from the miles, obviously you would do it by scratching the long division into the earth with a stick 🙂

How robust is plaster of paris? I want to make a thing to go on a wall. Art, if you will.

Will it crumble as soon as anyone looks at it?

How robust is plaster of paris?

Pretty stout if you do it well. Many a plaster wall plaque has survived for decades. Build it up in layers if it's going to be thick, and add fabric (plaster gauze) as a fibre reinforcement in any areas where you're not bothered about a fine surface finish

New (to me) frame has thrown up this issue. Not enough clearance at the drive side chainstay for the spider of the SRAM Omnium cranks to clear. Any suggestions? I've tried a thin spacer between the BB cup and the BB shell but it's not quite enough. Any more spacers and the NDS crank arm won't be threaded on to the spindle properly.

Is the best bet to try and dimple the chain stay? Is a double dimple, one on the outside, one on the inside of a tube, a bad idea?

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put new bullets on fired cases

Or, in the case of five of the .303" rounds we found among my father's effects, they pull the bullets and dump the powder out of live rounds then put the bullets back in, leaving the un-struck primer in place. Had us a little concerned until one of the bullets flopped out (no crimp, obvs.) revealing an empty case. The stripper clip was there too, so the whole lot must have fallen out of somebody's pocket. Quite the trophy for a young lad whose friends were making do with whatever spent brass fell from the dogfights overhead.

Not enough clearance at the drive side chainstay for the spider of the SRAM Omnium cranks to clear

That's quite common, the Omnomnoms combine a narrow (track) chainline with a fat spider. Some people dimple the stay, some switch to a more svelte crank.

I’ve heard similar things in safety lectures where someone fires a round with just a primer which blocks the barrel . They then fire a full power round bursting the barrel. Burst barrels look like bananas like in cartoons.

SRAM you bastards. I've had the same cranks for probably 10 years and never had this problem before. Being 10 years old, they're battered and probably not worth a huge amount (and I quite like them) so would prefer not to sell and replace them. How likely am I to die in a fiery blaze of glory/hubris if I try and dimple the chainstays myself?

had the same cranks for probably 10 years and never had this problem before

They definitely fit some frames, but you're far from the first to find a frame they don't fit. If the cranks don't owe you anything and the chainring bolts clear the stay, I'd probably file the cranks rather than putting a spider clearance dimple opposite that already huge tyre clearance dimple

Next stupid crank question. I have a 105 crankset in my parts box, I can't remember the exact model but it's about 5 years old. Is there anything to stop me slapping a 110BCD 1/8" chainring on there and riding it fixed? Is the chainline likely to be an issue?

Assuming Hollowtech?. Chainline will be something like 43.5mm I think, which if it's a double will be halfway between the two rings, so you could put the chainring on the inside and I think that would make the chainline 41mm. You could then space it out with a BB spacer to get it bang on 42mm (if thats what you're after)

That sounds right in my head, but someone with actual chainline chops might be able to confirm.

Yeah, Hollowtech! Sounds promising, might as well mock it up to test it out.

I made a chainline calculator on Excel once upon a time. I vaguely recall deleting it in a fit of self-disgust, though.

I need to 3D model a few things quickly (next few days). I used to use SketchUp and Vectorworks in 3D (10+ years ago). Lots of 2D Autocad experience but almost zero 3D. Can open a Rhino file but that's about it.

Is SketchUp web any good? Pro is expensive. I think I can get 3D vectorworks for free but struggling with the verification. I have access to SolidWorks but have never used it. Any recommendations?

Models will be simple elements (cuboids or extruded) no curves or complex forms, but positioned at complex angles.

Is the chainline likely to be an issue?

It's not going to line up with most track hubs. Your choices on a HT2 road double with a 1/8" ring end up being about 39mm inner and 48mm outer. The latter works well with a 6-bolt sprocket on a modified Boost (110×15) front wheel. You end up needing a lathe to move most common track hubs sideways, so the MTB hub option is no more trouble.

This all sounds far too complicated and time consuming, haha. If I can't make it work I might just have to buy a whole new crankset. The build of this £80 frameset is starting to look quite expensive!

far too complicated and time consuming

For a beater, using the inner ring position with 3/32" chain ring and sprocket and an 8-speed dérailleur chain is close enough.

A small extra dimple couldn't hurt much could it?
Edit: Although catching up on testers proposal/proposition/insert malapropism to file the rear tab of the cranks sound much better

Tinkercad might do you well.

https://www.tinkercad.com/

I had a 1/8" chainring mounted on the outer position of a HT2 set of cranks, possibly a small spacer behind the rear sprocket and never encountered any problems.

I might as well give it a go and see how close I can get to a decent chainline before spending anything. It's for a beater/commuter so doesn't need to be perfect. Good enough will do!

Surly recommends a 3/32 chain for anything other than a 100% chainline. You can get away with quite a bit of ‘tolerance’ as long as the ring and cog aren’t ramped.